feat: make Ollama/gemma4 the primary backend with Claude as fallback

Rolls back the claudification backend preference: PREFER_CLOUD_BACKEND now
defaults to false, resolve_backend() picks Ollama first and uses Claude when
explicitly preferred or when the new Ollama startup health check fails. The
Steward retries mid-request failures on the other backend in both directions.

Also hardens the fallback itself: Anthropic SDK imports are lazy so a broken
anthropic package degrades to Ollama-only instead of crashing at import time
(root cause of the production outage since April), anthropic is pinned to a
pydantic-ai-1.27-compatible range, ANTHROPIC_MODEL defaults to claude-sonnet-5
(sonnet-4-20250514 retired 2026-06-15), sampling parameters are stripped from
Claude calls (Sonnet 5 rejects them), and the Steward timeout is configurable
(STEWARD_TIMEOUT, default 60s) since gemma4 needs ~35s warm for analysis.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.env.example

@@ -8,17 +8,19 @@ API_HOST=0.0.0.0
 API_PORT=8000
 API_PREFIX=/v1
 
-# Anthropic Configuration (Claude - preferred backend)
-# Set ANTHROPIC_API_KEY to enable Claude as the default backend
-# Without an API key, Tatlock uses Ollama exclusively
-# ANTHROPIC_API_KEY=sk-ant-api03-your-key-here
-ANTHROPIC_MODEL=claude-sonnet-4-20250514
-PREFER_CLOUD_BACKEND=true
-
-# Ollama Configuration (local fallback when Claude unavailable)
+# Ollama Configuration (local - primary backend)
 OLLAMA_HOST=http://localhost:11434
 OLLAMA_DEFAULT_MODEL=gemma4:e2b
 OLLAMA_TIMEOUT=120
+STEWARD_TIMEOUT=60
+
+# Anthropic Configuration (Claude - cloud fallback)
+# Set ANTHROPIC_API_KEY to keep the Claude fallback available: it is used
+# automatically when Ollama is down, or exclusively when PREFER_CLOUD_BACKEND=true
+# Without an API key, Tatlock uses Ollama only
+# ANTHROPIC_API_KEY=sk-ant-api03-your-key-here
+ANTHROPIC_MODEL=claude-sonnet-5
+PREFER_CLOUD_BACKEND=false
 
 # SearXNG Configuration
 SEARXNG_HOST=http://localhost:8087